The Key Largo cotton mouse is a subspecies of the cotton mouse and is found on Key Largo in the upper Florida Keys. It is a slightly larger mouse with a more reddish color than other mouse species from peninsular Florida. Key Largo cotton mice can breed throughout the year and have an average life expectancy of 5 months. While the cotton mouse is one of the most common small mammals in South Florida, the Key Largo cotton mouse subspecies is endemic to Key Largo, and currently restricted to the northernmost part, due to habitat destruction. The Key Largo cotton mouse is currently on the United States Fish and Wildlife Service list of endangered species, due to urbanization of Key Largo. This cotton mouse subspecies can reach a body length of seven inches with a tail length of 3.0 in . Key Largo cotton mice have a dark hazel back with reddish-brown sides, a white belly, white feet, and a tail that is brown on top and white on the bottom. It is on the Fish and Wildlife Service's list of endangered species, due to the urbanization of Key Largo. The species previously resided in a variety of tropical hardwood habitats, but urbanization has decimated these areas, reducing the availability of food, shelter, and habitat for the cotton mouse. Surveys show that the Key Largo cotton mouse has lost up to 50% of its habitat due to this urbanization. The remaining habitat of the Key Largo cotton mouse is fragmented, making reproduction and population growth more difficult. Urbanization resulted in many other threats to the Key Largo cotton mouse, such as domesticated animals, human trash, and climate factors . Suggested actions to support the growth of the Key Largo cotton mouse populations and downgrade its listing from endangered to threatened include expanding public awareness and limiting pollution in the area.

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